PREHEAT oven to 350°F. Cook and stir onions in large skillet until crisp-tender. Add chicken, barbecue sauce, water and cumin; simmer until heated through, adding more water as necessary to keep the sauce from becoming too thick.

LAY 1 tortilla on a lightly greased foil-lined baking sheet, letting foil hang over sides of baking pan for tenting. Cover with half of the chicken mixture and half of the cheese. Top with second tortilla. Repeat chicken/cheese layers. Cover with the last tortilla; top with any remaining sauce. Sprinkle lightly with additional cheese, if desired. Tent with ends of foil.

BAKE 15 min. or until heated through. Allow to stand 5 min. before cutting into 6 wedges. Serve with sour cream.